Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy. These materials have been known in the art for many years and generally comprise a support having disposed thereon one or more imaging layers comprising (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing agent composition (acting as a developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the energy or image to another material.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. The preferred non-photo-sensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver of the silver carboxylate is reduced by a reducing agent whereby a black-and-white image of elemental silver is formed.
Problem to be Solved
Direct thermographic materials are imaged by contacting them with the thermal printhead of a thermographic recording apparatus such as a thermal printer or thermal facsimile. In such materials, an outermost protective layer is disposed over the imaging layer(s) to prevent damage to the soft emulsion layer, and to prevent sticking to, and to facilitate movement of the thermographic material during contact with the thermal printhead in the imaging apparatus. The thermographic material is then imaged at an elevated temperature, typically in the range of from about 300 to about 400° C. for 50 ms or less, to form a visible image [see for example U.S. Pat. No. 5,759,953 (Defieuw et al.)]. Such protective layers are also known in the art as transport or “slip” layers.
The outermost protective layer of the thermographic materials should be designed to provide smooth transfer into and out of the imaging apparatus under a wide range of imaging (or printing) conditions. Variable print forces along with the length or width of the material and variations in imaging temperatures can cause imaging defects. A deficiency in the performance of the protective layer causes intermittent rather than continuous transport across the thermal printhead.
Various lubricants and matting agents have been incorporated into the outermost protective layers of direct thermographic materials to solve the noted problems. For example, U.S. Pat. No. 5,759,953 (Defieuw et al.), U.S. Pat. No. 5,817,598 (Defieuw et al.), U.S. Pat. No. 5,536,696 (Horsten et al.), EP 0 669 875A1 (Horsten et al.), and EP 0 775 595A1 (Defieuw et al.) describe such components and their use in thermographic materials.
Many of the known lubricants used in thermographic materials are silicone-based lubricants that, while providing excellent protective or transport characteristics, have the disadvantage of providing a slippery feel to the outer surface when handled. Silicone based lubricants are also likely to transfer to the back of stacked films or roll materials, resulting in a loss of lubrication with time. In addition, this mobility at room temperature leads to the formation of ghost images when the thermographic materials are imaged. There has been considerable effort in the industry to find lubricants that solve all of the noted problems without causing other disadvantages.